This invention relates to a gas laser with an optics system and with a mode-masking diaphragm in the beam guiding chamber of the laser resonator.
Lasers of this type have been previously known and are described, for example, in EP 0 492 340.
A gas laser usually generates in its laser resonator a characteristic oscillation state, called the mode, which is essentially determined by the length of the laser resonator, the diameter of the laser tube(s) and the configuration of the electrodes. The design of a laser, and thus the type of mode it generates, depends on the intended application. For materials processing there are two modes of particular significancexe2x80x94the TEM00 mode (the so-called xe2x80x9cGaussian modexe2x80x9d) and the TEM01* mode (the so-called xe2x80x9cring modexe2x80x9d).
The Gaussian mode permits focussing down to the smallest possible spot diameter, a desirable feature for cutting thin sheet metal. The ring mode is more difficult to focus than the Gaussian mode, and generally results in a larger diameter for the focal spot. This is desirable for instance when cutting thicker sheet metal since the cutting width is large enough to permit the dross to be blown out. At the center of the ring mode beam, there is a power minimum, reducing the thermal load in the center of the optical elements, a feature which is important, particularly in the case of high-power systems.
Numerous attempts have been made in the past to set the mode in the laser resonator of a gas laser in a defined manner. The design described the above-mentioned EP 0 492 340 employs as its mode-masking diaphragm, two longitudinally adjustable aperture disks in the laser resonator by which the diameter of the laser beam can be reduced. In an initial setting, the two mode selector disks are positioned outside the laser beam, allowing the full diameter of the beam to exit unobstructed to the outside mirror. In a second setting, both mode diaphragm disks protrude into the beam path, reducing the diameter of the laser beam by about one half. A mechanically complex cylinder drive serves to move the mode diaphragm disks in the longitudinal direction.
In contrast thereto, it is the object of this invention to provide a novel gas laser with a relatively simple structure to enable simple switching between two different modes for different applications.
It has now been found that the foregoing and related objects may be readily attained in a gas laser having an optics system and a mode-masking diaphragm in the beam guiding chamber of the laser resonator. The optics system includes at least two adaptive optical elements adjustable between two settings, and the mode-masking diaphragm has an aperture which is disposed between the adaptive optical elements which are selectable between either of two settings, in each of which the mode-masking diaphragm masks out any higher-order modes from the laser beam.
Desirably, in at least one of the two settings of the adjustable optical elements of the optics system, one adaptive optical element serves to expand the laser beam while another adaptive optical element performs the subsequent focussing of the laser beam. Preferably, in at least one of the two settings of the optics system, two adaptive optical elements serve to expand the laser beam while a third adaptive optical element (32) focuses the laser beam.
The adaptive optical elements are selected from the group comprising the output mirror of the laser resonator, the retro-mirror of the laser resonator, and one or more interpositioned beam deflectors.
The function of the mode-masking aperture is provided by the inner diameter of a circular cross section of the beam guiding chamber. In one embodiment, the circular section of the beam guiding chamber is provided by one or more laser tubes of the laser resonator. In another embodiment, the circular section of the beam guiding chamber is provided by a connecting block linking two adjacent laser tubes. In one of the settings of the mode-masking diaphragm, the laser resonator is configured for generating a Gaussian mode and in the other setting the resonator is configured for generating a ring mode.
The laser also has a control device for setting the mode-masking diaphragm and the optics system, and at least two sets of parameters are stored in the control device for the two settings of the mode-masking diaphragm and optics system.
The advantage of the present invention lies in the fact that it makes it possible to generate in the laser resonator the Gaussian mode in one setting and the ring mode in the other setting, by a suitable control device which permits switching between the Gaussian and the ring mode as required for the intended application.
The switchable optics system preferably incorporates at least one adaptive optical element such as an adaptive mirror whose reflective surface can change shape for instance when the pressure of the cooling water is varied. This allows for the appropriate selection of the respectively desired curvature of the adaptive mirror by means of a control device.
The term xe2x80x9cadaptive optical elementxe2x80x9d as employed herein refers to mirrors and other optical elements of the laser assembly which can be modified in configuration or optical properties through the controlled application of external forces such as those which would change the temperature water pressure, piezoelectric or mechanical forces. Illustrative of such devices is the temperature cooling water pressure controlled mirror illustrated and described in Giesen et al U.S. Pat. No. 5,020,895 granted Jun. 4, 1991.
In at least one of the two optical element settings, one or two adaptive optical elements, preferably convex, serve as the laser beam expander while another adaptive optical element, preferably concave, serves for the subsequent focussing of the laser beam.
For the adaptive optical elements it is possible to use elements that are already parts of the optical path in the laser resonator, for instance the output mirror and/or the retro-mirror of the laser resonator and/or one or several beam deflectors. If the retro-mirror is to be used for beam expansion or focussing, the fact must be taken into account that, usually, it already has a curvature to assure a stable laser resonator. The final curvature of the retro-mirror is then defined by simply adding the two curvatures together, with due consideration being given to the respective sign. Actively operable optical elements include, for instance, a minimum of two neighboring adaptive beam deflectors, possibly in conjunction with the retro-mirror. The preferred configuration of actively operable optical elements consists of three adaptive beam deflectors.
Experiments with a double-squared convolution resonator have revealed that with at least two optical elements particularly good results are obtained, meaning a laser beam in the Gaussian mode with a particularly high beam quality, when one of the optical elements, especially when convex, is designed to expand the laser beam while the other optical element, especially when concave, serves for the subsequent focussing of the laser beam. Experiments with a double-squared convolution resonator have also revealed that with at least three optical elements particularly good results are obtained, i.e. a laser beam in the Gaussian mode with a particularly high beam quality, when two of the optical elements, especially when convex, serve to expand the laser beam while the other optical element, especially when concave, serves to focus the laser beam. The radius of the one concave optical element is preferably smaller than each of the radii of the convex optical elements while the radii of the convex optical elements are essentially matched. Particularly good results, i.e., a laser beam in the Gaussian mode with an especially high beam quality, can be obtained when one convex beam deflector is located near the output mirror or when the output mirror is itself convex and the radii of the convex or concave mirrors are in the range from 10 m to 60 m.
In particularly preferred design versions of the invention, the mode-masking aperture consists in the inside diameter of a round section of the beam guiding chamber. The advantage of this concept is that, in contrast to an aperture disk, there is no excited laser gas outside the excitation area defined by the round section of the beam guiding chamber, i.e. outside the mode masking aperture, which might otherwise give rise to undesirable amplification. Consequently, a crisp Gaussian mode can be generated in the laser resonator. Given the adjustable optics, the laser beam can be expanded within the beam guiding chamber to a beam diameter of a magnitude where the inside diameter of the round section of the beam guiding chamber acts as the mode diaphragm for masking out higher-order modes.
In particularly preferred design versions of the invention, the laser resonator is so configured that in one setting of the mode diaphragm and the optics it generates a Gaussian mode while in the other setting it generates a ring mode. This concept permits the selective setting of the laser mode with the aid of beam-expanding and focussing optical elements in the laser resonator, in the process of which the laser beam is shaped by these optical elements in such fashion that the round section of the beam guiding chamber of the laser resonator itself functions as the mode-masking aperture when the laser beam is expanded. Depending on the intended processing operation, a suitable control device can switch between the Gaussian and the ring mode. To that end, at least two sets of parameters for the two settings of the mode diaphragm and the optics may be stored in the control device.
Other advantages offered by this invention are evident from the description and the drawing. According to the invention, the features referred to above and those explained further below may be applied individually or in any desired combination. The design versions described and illustrated are not to be viewed as a finite and final enumeration but rather as examples serving to explain this invention.